Interior permanent magnet (IPM) synchronous machines are commonly used as electric traction motors in hybrid electric and battery electric vehicles. IPM synchronous machines are energized via a DC power source, typically a rechargeable battery module, in conjunction with a current-controlled voltage source inverter. However, the permanent magnets used in the rotors of such machines can complicate the response taken by a motor controller to a detected motor or drive system fault.
For instance, at higher motor speeds, the rotating magnets can create a back electromagnetic force (EMF) voltage in the motor's stator windings. If switching within the voltage inverter is temporarily disabled in response to a detected fault, the back EMF voltage may cause diodes within the inverter to conduct, thereby allowing electrical current to flow back toward the battery module. This current flow condition is commonly referred to as an “UnControlled Generator” (UCG) state. UCG state characteristics may include the presence of a relatively large amount of regenerative braking torque acting on the machine, as well as conduction of significant electrical current back to the battery module.
To combat this result, as a fault condition remedial action, controllers of IPM-type synchronous machines may short each phase of the multi-phase machine to one or more other phase, such that electrical current may flow from phase to phase instead of flowing back to the battery module. In a three-phase machine, this remedial action is referred to as a “three-phase short.” At higher motor speeds, braking torque is relatively low, which is favorable for traction drive applications. The machine impedance will limit the motor currents during a three-phase short operation. Additionally, stator current approaches the characteristic current of the machine for most motor speeds.